Tuflehundon
.270 WIN
I have had my Mossberg JM Pro for a couple of years now and love the gun. You hear good and bad things about it, but most people don't seem to know what needs to be done to the the gun running. So I figured I'd post up what I have learned. Not so much wanting to get into the go faster mods with this post. Mainly want to help people who are having issues with the 930's.
There are 3 things you need to do to get a JM Pro running right. They are all very simple to do, and can be done by just about anyone if you have a few basic tools. (this is assuming something isn't broken on the gun)
1. 1st thing you should do is throw away the follower that comes with the JM Pro and get something like the GG&G follower. The stock follower works great until you add an extension to the tube. Then it sucks. It will get caught up on the edge were the 2 tubes meet as it will always be a slight difference in size. The GG&G follower has a beveled front edge that keeps it from catching. The 1st 3 or 4 rounds will feed fine (depending on if you have the 22" or 24" gun), then it will seem as if there is no pressure on the shells. This is because the stock follower is caught up at the junction.
The newer guns have a different follower than mine did, but it still sucks.
2. Grind flat the front end of the recoil spring that is in your stock. I used a dremel with a grinding wheel to grind it flat. This will prevent it from digging into you recoil spring plunger which it can do very quickly. If you have burrs on your plunger, then you should also round off the rear edge of it. I put the plunger in my drill, turned it and and ran it against a file to round it off. These burrs can catch on the sides of the recoil spring tube and slow the gun down, causing misfeeds, failure to feeds, failure to eject, etc. My gun ran great until about 1200 rounds then started having big issues. This was the cause of most of my problems. This can also happen very quickly with the 930's. Helped a friend tune his gun last month that was new. he had less then 200 rounds through it and his plunger was starting to burr already. My gun just seemed like it ran slow. Here is a slow motion video of my gun when it started acting up.
Here is a pic of what my recoil spring plunger looked like after 1200 rounds.
I was also having the next issue, which is the feed from the tube problem, but wasn't able to diagnose it till I fixed the recoil spring plunger.
Edit: I will add that since posting this I have recieved a new plunger from OR3GUN. The replacement plunger fixes this problem all together. I would now recommend just replacing your plunger with the OR3GUN version. SO much better of a design than the stock one.
3. Polish the front edge of the carrier latch. This is the part that releases shells from the tube as the gun cycles, or you press the release. It can be sharp, and will catch on the ammo and cause the gun not to feed. If you are having problem getting the 1st round to feed from the tube when it's full, you are having this problem. Took me 10 minutes to round the edges off with a flat file. This can happen with some guns out of the box, or like mine after 1200 rounds. It doesn't happen with all guns, but it's such an easy fix that I would do this anyways. Brownells has a good tutorial on it here:
http://www.brownells... ... be-Problem
95% of the problems I have seen with these guns over the past couple of years has had to do with 1 of these things, or a combination of them.
There are 3 things you need to do to get a JM Pro running right. They are all very simple to do, and can be done by just about anyone if you have a few basic tools. (this is assuming something isn't broken on the gun)
1. 1st thing you should do is throw away the follower that comes with the JM Pro and get something like the GG&G follower. The stock follower works great until you add an extension to the tube. Then it sucks. It will get caught up on the edge were the 2 tubes meet as it will always be a slight difference in size. The GG&G follower has a beveled front edge that keeps it from catching. The 1st 3 or 4 rounds will feed fine (depending on if you have the 22" or 24" gun), then it will seem as if there is no pressure on the shells. This is because the stock follower is caught up at the junction.
The newer guns have a different follower than mine did, but it still sucks.
2. Grind flat the front end of the recoil spring that is in your stock. I used a dremel with a grinding wheel to grind it flat. This will prevent it from digging into you recoil spring plunger which it can do very quickly. If you have burrs on your plunger, then you should also round off the rear edge of it. I put the plunger in my drill, turned it and and ran it against a file to round it off. These burrs can catch on the sides of the recoil spring tube and slow the gun down, causing misfeeds, failure to feeds, failure to eject, etc. My gun ran great until about 1200 rounds then started having big issues. This was the cause of most of my problems. This can also happen very quickly with the 930's. Helped a friend tune his gun last month that was new. he had less then 200 rounds through it and his plunger was starting to burr already. My gun just seemed like it ran slow. Here is a slow motion video of my gun when it started acting up.
Here is a pic of what my recoil spring plunger looked like after 1200 rounds.
I was also having the next issue, which is the feed from the tube problem, but wasn't able to diagnose it till I fixed the recoil spring plunger.
Edit: I will add that since posting this I have recieved a new plunger from OR3GUN. The replacement plunger fixes this problem all together. I would now recommend just replacing your plunger with the OR3GUN version. SO much better of a design than the stock one.
3. Polish the front edge of the carrier latch. This is the part that releases shells from the tube as the gun cycles, or you press the release. It can be sharp, and will catch on the ammo and cause the gun not to feed. If you are having problem getting the 1st round to feed from the tube when it's full, you are having this problem. Took me 10 minutes to round the edges off with a flat file. This can happen with some guns out of the box, or like mine after 1200 rounds. It doesn't happen with all guns, but it's such an easy fix that I would do this anyways. Brownells has a good tutorial on it here:
http://www.brownells... ... be-Problem
95% of the problems I have seen with these guns over the past couple of years has had to do with 1 of these things, or a combination of them.
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